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Daniel R. Coates, Jeremy M. Chin, Susana T. L. Chung; Acuity, contrast, eccentricity, and crowding. Journal of Vision 2013;13(9):567. doi: https://doi.org/10.1167/13.9.567.
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© ARVO (1962-2015); The Authors (2016-present)
It is well known that acuity is impaired by factors such as reduced stimulus contrast, viewing eccentricity, and interference from nearby contours (crowding). Despite numerous studies, a comprehensive quantitative characterization of these effects, including any interactions, remains lacking.
We measured threshold letter size (acuity) for 150-ms tumbling-E targets at the fovea and lower visual field at eccentricities of 3, 5, and 10° for 5 subjects. Targets were presented in isolation or flanked on all four sides by randomly-oriented tumbling-Es at six center-to-center spacings (1.2-5x the letter size). Weber contrast of the entire stimulus varied from -2.5% to -99% in eight logarithmic steps.
We find that across subjects, contrasts, and eccentricities, flanked acuity can be well described as a function of nominal flanker distance. A parsimonious characterization is a two-line fit in log-log space comprising a flat portion where flankers do not affect target recognition, and, within the absolute critical spacing for crowding, a line with constrained slope of -1. Except where crowding is essentially absent (foveally), average r[sup]2[/sup] for this fit is 0.85±0.17. Interestingly, contrast reduction causes only a modest (<1.5x) increase in the critical spacing, instead primarily elevating the flat portion of these curves.
For all combinations of contrasts and flanker spacings, threshold acuities are linear functions of eccentricity (average r[sup]2[/sup]=0.94±0.069). The critical spacing is also linearly related to eccentricity (average r[sup]2[/sup]=0.98±0.038). Finally, a two-line fit relating acuity to contrast is consistent across subjects and eccentricities (average r[sup]2[/sup]=0.88±0.048), yielding a power-law with exponent of -0.55 for contrasts below approximately 20%.
Contrast-dependent flanked acuity for 224 conditions can be parameterized with just three per-subject values: baseline acuity, high-contrast flanked E2 and high-contrast unflanked E2. Except for slight increases in the crowding zone for low-contrast stimuli (at 2.5%, 1.5x greater than at 99%), crowding and contrast limit acuity independently.
Meeting abstract presented at VSS 2013
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